This invention relates to a marine propulsion system, and more particularly to such a system employing counterrotating propellers.
It has been found that providing a single propeller for a marine propulsion system results in imbalanced propeller torque leading to hydrodynamic inefficiencies. Utilization of dual counterrotating propeller substantially reduces or eliminates such inefficiencies.
The present invention discloses a marine propulsion system utilizing dual counterrotating propeller, along with a chain drive system for driving the propellers. The marine propulsion system according to the present invention comprises first and second concentric propeller shafts mounted to a depending gearcase, with each propeller shaft having a portion projecting from the gearcase. A first propeller is mounted to the projecting portion of the first propeller shaft, and a second propeller is mounted to the projecting portion of the second propeller shaft. A chain drive drivingly interconnects the first and second propeller shafts with the engine crankshaft. The chain drive includes a first portion for driving the first propeller shaft and a second portion for driving the second propeller shaft. Counterrotation drive means is provided for driving the first and second portions of the chain in opposite directions, thereby providing rotation of the first and second propeller shafts in opposite rotational directions.
In one embodiment, the counterrotation drive means includes one or more intermediate shafts carrying first and second upper sprockets. The first portion of the chain drive is provided about the first upper sprocket and a lower sprocket mounted to either the first or second propeller shaft. The second portion of the chain drive is provided about the second upper sprocket and a lower sprocket mounted to the other of the propeller shafts. The counterrotation drive means causes the first and second portions of the chain drive to be driven in opposite directions, resulting in counterrotation of the first and second propeller shafts. An input shaft, driven by the engine crankshaft, is rotatably disposed in the upper portion of the depending gearcase. The counterrotation drive means includes a first drive means provided between the input shaft and the first upper sprocket for driving the first upper sprocket in a first rotational direction, thereby driving the first portion of the chain drive in a first direction. The counterrotation drive means further includes a second drive means disposed between the input shaft and the second upper sprocket for driving the second upper sprocket in a second rotational direction opposite that of the first upper sprocket, and driving the second portion of the chain drive in a direction opposite that of the first portion of the chain drive. In one embodiment, the first drive means comprises an input gear connected to and rotatable with the input shaft, and engageable with a gear connected to and rotatable with the first upper sprocket. The second drive means comprises an input sprocket connected to and rotatable with the input shaft, and a sprocket connected to and rotatable with the second upper sprocket. A drive chain is provided about the input sprocket and the sprocket connected to the second upper sprocket. With this arrangement, rotation of the input shaft results in rotation of the first and second upper sprockets in opposite rotational directions. The first and second portions of the chain drive are thus driven in opposite directions, resulting in rotation of the dual propellers in opposite directions.
Various other embodiments are disclosed for imparting counterrotation to the first and second upper sprockets, and thereby opposite directions of movement of the first and second portions of the chain drive.